Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A technician is troubleshooting a three-phase motor that consistently fails to start, producing only a humming sound, and drawing excessive current. The motor's thermal overloads trip almost immediately. All supply voltages are balanced and correct. Which of the following is the most likely cause of this issue?
- ASingle-phasing condition in the motor circuit.
- BOverloaded motor due to mechanical binding.
- CMotor winding insulation breakdown.
- DIncorrect overload relay setting.
Show answer & explanationAnswer & explanation
Correct answer: A. Single-phasing condition in the motor circuit.
A three-phase motor operating on only two phases (single-phasing) will typically hum loudly, fail to start or run at reduced power, and draw excessive current in the remaining two phases. This excessive current will quickly cause the thermal overloads to trip, protecting the motor from severe damage. Balanced supply voltages rule out upstream supply issues, pointing to an issue within the motor circuit itself.
Why the other options are wrong
- B. While an overloaded motor would draw excessive current and trip overloads, it would usually attempt to start or run, not just hum and fail to start with correct voltages.
- C. Motor winding insulation breakdown often leads to ground faults or short circuits, which would typically trip branch-circuit protection (breakers/fuses) rather than just thermal overloads, and might present with smoke or more severe symptoms.
- D. An incorrect overload setting would cause nuisance tripping, but if the motor hums and fails to start while drawing excessive current, it indicates a motor fault rather than just a protective device misconfiguration.
Single-Phasing in Three-Phase Motors
A condition where a three-phase motor loses power to one of its phases, causing it to hum, fail to start or run weakly, and draw excessive current in the remaining phases, leading to rapid overload tripping.
- Motor operates on only two phases.
- Causes unbalanced currents.
- Leads to overheating of windings.
- Overloads trip quickly to prevent damage.
Memory trick: A hum and no start, means a phase is apart.